/**
 * The interface for the consumer of an observable sequence
 */
export interface Observer<T> {
    /**
     * Called for the next element in the sequence
     */
    next: (value: T) => void;

    /**
     * Called once when the error occurs in the sequence.
     */
    error: (e: unknown) => void;

    /**
     * Called once at the end of the sequence.
     */
    complete: () => void;
}

/**
 * The group of functions to feed an observable. This methods are provided to
 * the producer to generate a stream of events.
 */
export type Sink<T> = {
    [k in keyof Observer<T>]: (this: void, ...args: Parameters<Observer<T>[k]>) => void;
};

export type Producer<T> = (sink: Sink<T>) => (void | (() => void));

export interface Unsubscribable {
    unsubscribe(): void;
}

export const isUnsubsribable = (v: unknown): v is Unsubscribable =>
    v !== null && v !== undefined && typeof (v as Unsubscribable).unsubscribe === "function";

export const isSubsribable = <T = unknown>(v: unknown): v is Subscribable<T> =>
    v !== null && v !== undefined && typeof (v as Subscribable<unknown>).subscribe === "function";

export interface Subscribable<T> {
    subscribe(consumer: Partial<Observer<T>>): Unsubscribable;
}

/** The observable source of items. */
export interface Observable<T> extends Subscribable<T> {
    /** Transforms elements of the sequence with the specified mapper
     * 
     * @param mapper The mapper used to transform the values
     */
    map<T2>(mapper: (value: T) => T2): Observable<T2>;

    /** Filters elements of the sequence. The resulting sequence will
     * contain only elements which match the specified predicate.
     * 
     * @param predicate The filter predicate.
     */
    filter(predicate: (value: T) => boolean): Observable<T>;

    /** Applies accumulator to each value in the sequence and
     * emits the accumulated value for each source element
     * 
     * @param accumulator 
     * @param initial 
     */
    scan<A>(accumulator: (acc: A, value: T) => A, initial: A): Observable<A>;
}

const noop = () => { };

const sink = <T>(consumer: Partial<Observer<T>>) => {
    const { next, error, complete } = consumer;
    return {
        next: next ? next.bind(consumer) : noop,
        error: error ? error.bind(consumer) : noop,
        complete: complete ? complete.bind(consumer) : noop
    };
};

const fuse = <T>({ next, error, complete }: Sink<T>) => {
    let done = false;
    return {
        next: (value: T) => { !done && next(value); },
        error: (e: unknown) => { !done && (done = true, error(e)); },
        complete: () => { !done && (done = true, complete()); }
    };
};

const _observe = <T>(producer: Producer<T>): Observable<T> => ({
    subscribe: (consumer: Partial<Observer<T>>) => ({
        unsubscribe: producer(sink(consumer)) ?? noop
    }),
    map: (mapper) => _observe(({ next, error, complete }) =>
        producer({
            next: next !== noop ? (v: T) => next(mapper(v)) : noop,
            error,
            complete
        })
    ),
    filter: (predicate) => _observe(({ next, error, complete }) =>
        producer({
            next: next !== noop ?
                (v: T) => predicate(v) ? next(v) : void (0) : noop,
            error,
            complete
        })
    ),
    scan: (accumulator, initial) => _observe(({ next, error, complete }) => {
        let _acc = initial;
        return producer({
            next: next !== noop ?
                (v: T) => next(_acc = accumulator(_acc, v)) : noop,
            error,
            complete
        });
    })
});

export const observe = <T>(producer: Producer<T>): Observable<T> => ({
    subscribe: (consumer: Partial<Observer<T>>) => ({
        unsubscribe: producer(fuse(sink(consumer))) ?? noop
    }),
    map: (mapper) => _observe(({ next, error, complete }) =>
        producer(fuse({
            next: next !== noop ?
                (v: T) => next(mapper(v)) : noop,
            error,
            complete
        }))
    ),
    filter: (predicate) => _observe(({ next, error, complete }) =>
        producer(fuse({
            next: next !== noop ?
                (v: T) => predicate(v) ? next(v) : void (0) : noop,
            error,
            complete
        }))
    ),
    scan: (accumulator, initial) => observe(({ next, error, complete }) => {
        let _acc = initial;
        return producer(fuse({
            next: next !== noop ? (v: T) => next(_acc = accumulator(_acc, v)) : noop,
            error,
            complete
        }));
    })
});